It was eight o’clock on the morning of June 11, 1996. I was only two months old that day my forty-eight year-old grandfather was found face-down on his bed, lifeless. “What had happened?” The question arose in every mind. When the autopsy test results were returned, the family was shocked with the news that my grandfather had been taken by a heart attack.
Twelve years later, at six o’clock on a Sunday evening, my other grandfather was told that he was suffering from his first heart attack. After hospitalization, he underwent the Percutaneous Coronary Intervention operation, which entails angioplasty and the placement of at least one stent (in his case, it was three). On November 9, 2012, he was hospitalized again, and survived his third coronary surgery.
Why does one of my grandfathers still live and not the other, even though they both suffered from a heart attack? It was not the severity of the heart attack; the difference was the application of technology. My first grandfather lived in a tiny town of one hundred people, ninety miles from the nearest hospital; the second lives in the medical center of the state. The first did not seek medical help, but the second did, and the placements of nine stents and three angioplasties over the past five years have allowed him to continue living. So, one might ask, what is this miracle operation, and what about this coronary treatment has made it work effectively?
Although there are various engineering aspects connected to this procedure, including x-ray and catheter technology this paper will focus on the creation, current use, and possibilities for the future of the stent. In 1977, Dr. Andreas R. Gruentzig, the doctor who invented the catheter, performed the first angioplasty in Switzerland. A coronary angioplasty is performed, just as it was in 1977, to arteries narrowed by thrombosis (plaque build-up or blood clots that restrict or stop blood flow) causing coronary artery disease or a myocardial infarction (heart attack). A balloon on the end of a catheter (a thin tube) is guided to the blockage site, where the balloon is inflated for up to two minutes in order to force the plaque or clot to the sides of the artery wall, thus clearing the passageway for blood to flow freely once again. In the beginning, there was only a 50-70% success rate because fatty plaque would relocate, blocking the artery again within a few months. To solve this problem, biomedical and material engineers began to design stents, structures with a shape similar to a sleeve, to hold plaque to the walls of the arteries and to hold the artery walls open at the natural diameter. In 1994, cardiologists began to use the newly introduced stainless steel metal stents in operations alongside the use of the angioplasty, but soon found that “[t]he very process of widening the artery traumatizes the artery wall and the body reacts by setting up an inflammatory response. The body, recognizing the stent as an unwelcomed foreign body, overcompensates in repair to the injured site” (Wilde, 198-199). Resulting scar tissue accumulates within the artery, causing a condition called restenosis, where an artery is reclogged after treatment. In order to reduce restenosis, biomedical and chemical engineers with Johnson and Johnson (also known as Cordis) designed the medicated stent, a metal sleeve coated in medication to reduce the body’s production of scar tissue. However, the reduction of restenosis was minimal, and a better stent was still sought after by cardiologists. In 2003, the Food and Drug Association (FDA) approved a new answer, drug-eluting stents (DES), which gradually release medications over time. As author and Christian Wilde referred to it, DES became “the state of the art technology, the next generation of stent” (Wilde 200). Designed by biomedical and chemical engineers, DES emit medication over a period of time, drastically lowering shortterm restenosis rates. Largely successful, DES are commonly used by cardiologists, but mechanical engineer studies have shown that the structures most widely used (struts in the shapes of rectangles, circles, and trapezoids) disrupt the normal blood flow, which slightly increases restenosis chances in DES patients. Some studies also suggest that DES are more useful in preventing restenosis and thrombosis than in preventing more major conditions such as myocardial infarction. In 2006, study results indicating that DES increased blood clot and heart attack risks spread rapidly, but the FDA intervened by determining the risk so infinitesimal that it is insignificant compared to the benefits of DES.
Due to engineering inventions, discoveries and experiments, many different structures are used to design stents, and multiple components determine the effectiveness of the stent as a whole. Such components include strut thickness, density, manufacturing materials, design, and stent coatings. Cardiologists and engineers have collaborated to discover which components are most conducive to specific circumstances. Thicker stent struts were mechanically engineered to increase immediate performance but were found to decrease long-term results. In response, thinner struts and lower metal densities were incorporated, which resulted in lower restenosis risks, making them more useful in smaller arteries. Because it was discovered that meshwire stents do not work well in tortuous arteries; tubular and corrugated stents were designed and have proven to work better overall. Further research found that high density metal stent placements, such as those made of meshwire, are more likely to experience thrombosis and restenosis, so coil and hybrid stent designs were created to be more flexible and conforming to the artery shape. However, they have been discovered to be weaker in holding the artery wall open. Material engineers have concluded that cobalt-chromium is thinner than stainless steel and more radiovisible than most thin materials. Consequently, it is less conducive to restenosis and is easily seen on monitors during insertion, so it is now being used as a metal in stent manufacturing. Chemical engineers further found that metal stents coated in materials such as gold and silicon carbide do not decrease thrombosis and restenosis rates, and gold even increases them. However, stents covered in drugs (DES) have most successfully decreased the probability of restenosis in patients. Because of varying conditions, it is difficult for engineers to create an all-purpose stent design that works best for every situation, but they continue to try new materials and designs to improve upon their invention.
Stent projects in action for future application include designs based on fluid-dynamics and dissolvable stents. Although neither has been approved by the FDA, experiments have been performed concerning both stent variations. Created by biomedical engineers, fluid-dynamic stents, a relatively new idea with little experimentation, reflect medical scientists’ belief that the application of flowing fluid properties, specifically that of blood, can help mechanical and biomedical engineers design stents that reduce cell tissue build-up, keeping restenosis low. Likewise, engineers the world over have been attempting to design a stent that functions just as well as drug-eluting stents but dissolves into the artery well. This way, a foreign body does not reside within the artery, and DES effectiveness is not sacrificed. However, engineers have stumbled upon obstacles through patient experiments conducted in multiple European and Asian countries. Complications include the weakness of biodegradable polymers (the dissolve material) in comparison to metal stents, requiring thicker struts. In Japan, the Igaki-Tamai (first inserted biodegradable stent) was used in a ten-year study where 50 patients received the stent between September of 1998 and April of 2000. Because successful experimentation is required before deeming the stent acceptable for coronary use, the stent was used in leg arteries. Initial trials indicate outcomes similar to the use of bare metal stents but, unfortunately, took approximately three years to dissolve rather than the predicted six months. In 2006, the first patient received an Abbott (producer of stents) dissolvable stent, which was supposed to dissolve within two years. Results have been successful in initial experiments and indicate that the dissolvable stent will be a revolutionary product. Redesigned in 2009 by engineers seeking to improve dissolvable stents to further reduce restenosis and possible collapsing of the artery, it will not be used in the United States until successful long-term results have been collected. Other companies have been testing dissolvable stents on patients in several countries, including Germany and India, but there are currently no stents that have gathered enough successful longterm evidence to be considered for approval in the United States.
Although the coronary stent may seem rather minute and unimportant when compared to drastic operations such as bypass surgery or the placement of a pacemaker, stents play an important role in the cardiology world and have saved millions of lives, my grandfather being only one of them. According to Wilde, in 2006 Americans were receiving 850,000 stents per year. At this rate, in less than two and a half years, patients receive two million stents in the United States alone. The process of improving treatments such as the coronary stent is still not complete, for with the constant availability of new technology, engineers continually modify past inventions to increase success rates of medical treatments.
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